Generally, a concern of printing industry is printing quality control. For example, U.S. Pat. No. 7,054,568 entitled “Method and Apparatus for Controlling Non-Uniform Banding and Residual Toner Density Using Feedback Control” has addressed various quality problems. For example, in a xerographic printing system, printing quality can be affected by incomplete transfer of a toner image from a photoreceptor to an intermediate transfer belt or from the intermediate transfer belt to a paper. Because of some strongly adhering toner particles, low charge toner particles, air breakdown, or other reasons, the transfer of the image from the photoreceptor to the intermediate transfer belt or the paper, or from the intermediate transfer belt to the paper, can be incomplete. If toner transfer efficiency varies significantly, toner density on a final image may change. If the final image is a colored image, then changes in the toner density can result in color shift.
U.S. Pat. No. 7,054,568 proposes a technique using a feedback control system to address various quality control problems that may happen in a printing system. Generally, a feedback control system can include a working system, a monitoring system, and a controlling system. The monitoring system can monitor the working system, measure parameters of the working system and provide to the controlling system. The controlling system can analyze variations of the working system based on the parameters provided by the monitoring system. Accordingly, the controlling system can send instructions to adjust the working system. In order for the feedback control system to work well, the monitoring system needs to accurately measure parameters of the working system in real-time. However, due to limitations of the monitoring system, the environment noise, defects in the working system or other disturbance, it can be difficult for the monitoring system to measure accurately.